This invention relates to a thermal imaging system of the type comprising a telescope for gathering and directing radiation onto a cryogenically cooled detector.
Part of a typical thermal imaging system is shown in FIG. 1, and comprises an encapsulated cooled detector arranged to receive radiation from a lens system and scanner (not shown). The detector and its substrate are generally cryogenically cooled, often by liquid nitrogen, so as to reduce the level of intrinsic noise generated by the detector.
One problem with this arrangement is that the detector and substrate emit radiation characteristic of a low temperature which may be retro-reflected back by the elements of the telescope and the imager body and detected by the detector. The perceived low temperature is not representative of the image the apparatus is intended to view, and is often seen as a dark area in the centre of the image. Such an effect is known as narcissus and tends to degrade the image.
The factors that affect the degree of narcissus in a system are the curvature of the reflecting surfaces, the reflectivity of each surface and the size of the cold area visible to the detector, by reflection.
It is possible to reduce the reflectivity of the surfaces by multi-layer coatings, but no such coating is 100% efficient and a small percentage of radiation will inevitably be reflected. Since the detector and substrate are at a very low temperature, only a small fraction of the difference in radiation of the detector and its surroundings. When added to the temperature of the desired image, is sufficient to create a spurious temperature difference, or narcissus.